Effects of Moisture and Other Vapours

The effect on kinetic friction is less straightforward. At constant humidity there is a tendency for the coefficient of kinetic friction to decrease with time of sliding . [Pg.81]

Another theory of the reason for increased friction in the presence of moisture was proposed by Gao et al . They found that in a humid environment molybdenum disulphide films were more readily thinned by sliding contact, due to increased ease of interlamellar slip. They suggested that adsorption of water softened the films, and that resulting increased deformation by plowing in sliding contact led to a poorly oriented film and thus to increased friction. However, they considered that this was a short-term reversible effect which was not in conflict with theories of chemical breakdown. Gao et al also poiinted out the possibility that an increase In friction is caused by capillary pressure effects of moisture at asperity contacts. [Pg.83]

In one respect the effect of moisture is not reversible. Sliding in the presence of moisture has been shown to result in evolution of hydrogen sulphide gas , and this must be caused by irreversible chemical reaction, probably involving oxidation. It may therefore be the case that some part of the increase in friction in the presence of moisture is caused by an increase in the amount of surface oxide. If this is so, then it seems probable that this oxide is preferentially removed on further sliding, allowing the lubricant to revert to a low coefficient of friction in a dry atmosphere. [Pg.85]

Other polar vapours also cause a significant increase in the coefficient of friction , but the increase due to the vapour of non-polar compounds is relatively small. There is a lack of specific information on the effects of other vapours on film formation or wear life. Analogy with the effects of water vapour would again suggest that polar compounds would increase wear rate while non-polar compounds have little effect. [Pg.85]

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